The German patent publication DE-AS No. 29 18 974 describes a method of producing gypsum fiber boards in which pulp fibers are comminuted to a certain degree and mixed with calcium sulphate hemihydrate and possible further additives to a wet mass from which subsequently the boards are shaped.
Hardening of the boards is obtained by setting the hemihydrate to calcium sulphate dihydrate. This method has the drawback that the shaping of the boards must be carried out only during the short setting period of the hemihydrate. Moreover, this method renders it rather difficult to rouse excess material obtained for example during cutting of the boards.
The German publication DE-OS No. 26 49 300 describes a method for producing plate-shaped material in which calcium sulphate hemihydrate is mixed in the presence of water with 5-60 weight-% fibrous material. Subsequently, the pulp is dewatered and the formed cake is shaped to the desired size. Thereafter, the still moist plate-shaped material is dried at temperatures between 95.degree. and 140.degree. C. For improving the properties of the final product, a binding agent is added to the mixture of water, calcium sulphate dihydrate and fibrous material. Recrystallization of the gypsum is not provided. Gypsum fiber boards produced in this manner have considerably less strength in comparison to those obtained through recrystallization of hemihydrate to dihydrate.
The production of formed bricks is disclosed in the German publication DE-OS No. 31 17 662 in which moist calcium sulphate dihydrate--possibly mixed with sand or other aggregates--is pressed and then hydrothermally treated at a temperature of up to 200.degree. C. to provide a dehydration. The hydrothermal treatment is e.g. performed in an autoclave. Thereafter, i.e. after being removed from the autoclave, the shaped bodies are allowed to cool down.
An apparatus for producing gypsum fiber boards is known from the German publication DE-OS No. 32 28 159. Here, fibrous material and gypsum are supplied to a mixer to provide a suspension which is placed onto an endless screening belt for dewatering the suspension and then is transferred onto a further conveyor belt by means of a transfer device. After passing a cutting device, the formed boards are introduced into a dryer. This prior art apparatus is rather complicated and provides gypsum fiber boards of insufficient strength.